1. Field of the Invention
The present invention relates to a field of observing natural earthquake, and more particularly, relates to a combined broadband ocean bottom seismograph (OBS) with single glass sphere.
2. Description of the Related Art
Institute of Geology and Geophysics, Chinese Academy of Sciences is a major institution in China engaging in researching and developing ocean bottom seismograph and takes on most of tasks for developing ocean bottom seismograph in China. In the special development program financially supported by Ministry of Finance for developing important scientific research equipment during the Outline of the Eleventh Five-Year Plan of China, Institute of Geology and Geophysics, Chinese Academy of Sciences developed a seven-channel multifunctional ocean bottom seismograph, and has successfully put it into use in the South China Sea, the Yellow Sea and the Bohai Sea and has obtained a great amount of valuable scientific data.
FIG. 1 is a structural view of a conventional multifunctional ocean bottom seismograph with seven-channels. As shown in FIG. 1, the conventional multifunctional ocean bottom seismograph with seven-channels comprises a release mechanism, a plastic instrument chamber, a glass instrument sphere and an anchor. The release mechanism is provided at the top end of the plastic instrument chamber. The glass instrument sphere is fixedly mounted inside the plastic instrument chamber. The anchor is provided at the bottom of the plastic instrument chamber. The release mechanism and the anchor are connected with each other by tensioning steel wires so that the instrument chamber is secured in the anchor. Inside the glass instrument sphere is provided with a battery unit, an O-ring, a broadband three-component leveling seismometer, a three-component high frequency detector, a normally-horizontal support, electronic circuit, an acquisition system and a water acoustic communication module and the like. The battery unit is disposed in the lower half of the glass instrument sphere via the O-ring. The broadband three-component attitude control seismometer and the three-component high frequency detector are integrated with each other by screws to form a seismometer assembly. The seismometer assembly is connected to the normally-horizontal support inside the glass instrument sphere via bearings to rotate freely within a working angle of 30 degree. The electronic circuit, the acquisition system, and the water acoustic communication module are secured on the top of the seismometer assembly comprising the broadband three-component attitude control seismometer and the three-component high frequency detector to integrate into a unit inside the glass global chamber.
With the conventional multifunctional ocean bottom seismograph with seven-channels, the seismometer assembly is disposed inside the glass instrument sphere which is enclosed by the plastic instrument chamber, and the plastic instrument chamber is coupled to the anchor and the anchor is then coupled to the seabed. Due to such multiple coupling, the data recorded by the seismometer assembly are amplified by the plastic chamber, the anchor and other relevant components. Furthermore, it is difficult to ensure the sufficient coupling strength during assembly of these components, which adversely affects the seismic record and results in large error in obtained data.
In addition, Woods Hole Oceanographic Institution (WHOI) and Scripps Institution of Oceanography (SIO) develop a kind of ocean bottom seismograph, which mainly comprises a cantilever beam for hanging the seismometer so that the seismometer works on the seabed directly. In order to keep away from the anchor and floating ball, the cantilever beam must have a certain length, and to avoid external torque influence due to external condition, the seismometer is disposed in a separate glass sphere. To provide sufficient buoyancy, multiple spheres are necessary. Typically, there are four to eight glass spheres, so that the weight of the seismograph amounts to several hundreds kilogram or more, and the manufacture cost is increased greatly. The price of a single seismograph is nearly one million Renminbi (RMB). Further, it increases difficulty in putting the seismograph into operation under sea and recovering it from the ocean bottom. FIG. 2 shows a structure of the seismograph provided by WHOI.
Presently, with the higher requirements for performance of the ocean bottom seismograph, the ocean bottom seismographs developed in several countries and the broadband multifunctional ocean bottom seismograph with seven-channels previously developed by the applicant have some disadvantages: there is much difference in performance of the ocean bottom seismograph when it works in sea water environment and on land. The frequency of the natural seismic signal is in the range of 0.01-10 Hz. The coupling between the ocean bottom seismograph and the seabed has apparent influence on the data quality in seismic observation, and the surging of the sea water has great influence on the horizontal signal recorded by the instrument during work. Firstly, the less the pressure on the seabed applied by the ocean bottom seismograph, the better the coupling between the ocean bottom seismograph and the seabed. In other words, the more proximity between the density of the ocean bottom seismograph and the density of the sea water, the better the recording effect. Secondly, since the inconsistency between the vibration of the seabed and the motion of the sea water, the ocean bottom seismograph will be affected by additional moments when it is recording signals, and the higher the ocean bottom seismograph, the longer the equivalent arm of force, resulting in a greater influence.
For the above reasons, the current broadband multifunctional ocean bottom seismograph with seven-channels in which the seismometer is integrally mounted inside the glass instrument chamber cannot resist well the impact of sea water flow and there is great distortion of recorded signal. On the other hand, the broadband ocean bottom seismograph with a cantilever beam cannot be broadly used due to the weight and the cost.